1. Field of the Invention
The present invention relates to a digital signal reproducing apparatus and, more particularly, to an apparatus for detecting a predetermined pilot signal component from a reproduced signal.
2. Description of the Related Art
In recent years, with the advance of magnetic recording/reproduction technology, magnetic recording media and the like, it has become possible to record a large amount of digital information on a small-size magnetic tape or magnetic disk. The pitch of recording tracks on a recording medium has been becoming smaller and smaller for the purpose of higher-density recording, and the art of enabling a reproducing head to accurately trace such small-pitch tracks, i.e., tracking control, is an important technical subject to be considered in the field of digital signal reproducing apparatus.
An art for achieving such tracking control has been proposed. In the art, digital modulation is applied to multiplex a pilot signal component for tracking control with a digital signal to be recorded on a track, i.e., a redundancy bit is assigned to each digital data to be recorded and the redundancy bit is set to "1" or "0" to multiplex a pilot signal component of predetermined frequency (for example, f1 or f2) with the digital data.
FIG. 1 is a schematic block diagram showing the essential construction of a reproducing circuit for a digital VTR which is arranged to detect a pilot signal component multiplexed with a digital signal and perform tracking control.
As shown in FIG. 1, a magnetic tape 3 is wrapped around a rotary head drum 1 over an angular extent of 180.degree. or more, and the digital data recorded on a multiplicity of tracks formed on the magnetic tape 3 are sequentially reproduced by two rotary heads HA and HB which are arranged to rotate with a phase difference of 180.degree.. The outputs of the rotary heads HA and HB are respectively amplified by reproducing amplifiers 5 and 7, and the outputs of the reproducing amplifiers 5 and 7 are inputted to a switch 13.
The rotational phase of the rotary head drum 1 is detected by a PG head 9, and a head switching circuit (HSW circuit) 11 forms a head switching pulse (HSW) according to a detection signal (PG) indicative of the detected rotational phase. FIG. 2 is a schematic view showing a recording pattern which is formed on the magnetic tape 3, and FIG. 3 is a timing chart showing waveforms which are obtained at essential locations in the circuit shown in FIG. 1. As shown in FIG. 3, the HSW having a rectangular waveform is obtained from the HSW circuit 11.
The switch 13 operates according to the HSW, and supplies the output of the rotary head HA or HB which is tracing the magnetic tape 3 to the circuits provided at the next stage.
The signal outputted from the switch 13 has the envelope waveform shown in FIG. 3 and contains the shown pilot signal components. In FIG. 2, each track marked with f1 or f2 indicates that the pilot signal component having the predetermined frequency f1 or f2 is multiplexed with the track, and each track marked with f0 indicates that neither of the pilot signal components f1 and f2 is multiplexed with the track and the pilot signal components f1 and f2 are attenuated. Accordingly, the pilot signal components mainly contained in the signal outputted from the switch 13 are as shown in FIG. 3.
The output of the switch 13 is supplied to a reproduced signal processing circuit 15, in which the original digital information is detected. The reproduced signal processing circuit 15 performs predetermined processing, such as error correction and data decoding, on the original digital information, to restore the original digital video signal, and outputs the original digital video signal to an output terminal 17 as a reproduced video signal.
The output of the switch 13 is also supplied to band-pass filters (BPFs) 21 and 23, in which the pilot signal components f1 and f2 are detected, respectively. Tracking control is achieved during the reproduction of a digital signal from any of the tracks marked with f0, by making comparison between the pilot signal components f1 and f2 which leak from both adjacent tracks to the digital signal during such reproduction and controlling the amount of transport of the magnetic tape 3 so that the ratio of the pilot signal component f1 to the pilot signal component f2 can be made constant (normally, 1:1).
The pilot signal components f1 and f2 extracted by the BPFs 21 and 23 are respectively inputted to switches 25 and 27. As shown in FIG. 2, the positional relationship between both tracks adjacent to each of the tracks marked with f0 is inverted every other track in such a manner that the pilot signal components f1 and f2 are alternately multiplexed onto every fourth track. Accordingly, each of the switches 25 and 27 is switched at intervals of a two-track period in accordance with the signal obtained by frequency-dividing the HSW in a 1/2 frequency divider 19, whereby the pilot signal components f1 and f2 which leak to a track (f0) which is being traced from both adjacent tracks are supplied to detecting circuits 29 and 31. The output of the 1/2 frequency divider 19 is shown in FIG. 3.
The detecting circuits 29 and 31 detect the respective input signals and convert them into the corresponding voltage levels, and supply the voltage levels to a subtracting circuit 33. The subtracting circuit 33 makes comparison between the amounts of leaks from both adjacent tracks and inputs the comparison result to a switch 35 as a tracking error signal. Referring to the opening and closing timing of the switch 35, when the HSW of FIG. 3 is at its high level, the switch 35 is closed since any of the tracks marked with f0 is being traced for reproduction, whereas when the HSW of FIG. 3 is at its low level, since any of the tracks marked with f1 and f2 is being traced for reproduction, the switch 35 is opened to hold the previous voltage level.
The output of the switch 35 is smoothed by a loop filter 37, and the output of the loop filter 37 is inputted to an adding circuit 39 as a tracking error signal. A frequency generator (hereinafter referred to as the FG head) 45, which is attached to a capstan motor 47, outputs a pulse having a frequency proportional to the rotational speed of the capstan motor 47. The frequency signal (capstan FG) outputted from the FG head 45 is inputted to a speed loop servo circuit 41, which forms a speed control signal for adjusting the rotational speed of a capstan to a desired rotational speed.
The adding circuit 39 adds together the tracking error signal and the speed control signal, and applies the sum of these signals to a motor driver 43 as a final control voltage for the capstan motor 47. The capstan motor 47 is driven by the motor driver 43, whereby the magnetic tape 3 is transported in the longitudinal direction so that the rotary heads HA and HB can correctly trace individual tracks on the magnetic tape 3.
However, since the tracking control circuit (ATF circuit) of the above-described example is composed of an analog circuit, the passbands of the respective BPFs 21 and 23 become considerably low and narrow (some tens of kilohertz), so that an increase in the scale of the entire circuit construction is difficult to avoid.
If such an ATF circuit is to be digitized, a digital filter having an enormous number of taps must be constructed because the clock frequency of a digital signal (for example, 41.85 MHz) is considerably distant from the frequencies of the pilot signal components f1 and f2 to be extracted and, in addition, because each of the BPFs 21 and 23 has such a narrow bandwidth.
One approach is to perform subsampling before a digital BPF and lower the clock frequency to a great extent. To realize this approach, it is necessary to insert a prefilter for cutting off aliasing noise before subsampling, i.e., it is necessary to suppress frequency components of f.sub.s '/2 or higher (f.sub.s ' represents the subsampling frequency). However, if such a pre-filter is provided in this manner, the pre-filter itself needs a considerably large number of taps and the hardware scale becomes large.